1. Field of the Invention
The present invention relates generally to a high frequency power amplifier such as used in a mobile telephony device, and relates more specifically to a bias circuit and to a bias supply method for a multistage power amplifier of heterojunction bipolar transistors.
2. Description of Related Art
Heterojunction bipolar transistors (HBT), which operate with a single power supply, are replacing GaAs field effect transistors (FET) and GaAs high electron mobility transistors (HEMT) in devices used for power amplification of radio frequency (RF) signals in mobile telephones and other mobile communications products. GaAs FETs and GaAs HEMTs require a negative voltage supply to the gate, and therefore require a negative voltage generator in the mobile telephone or other device in which they are used. Compared with a conventional FET, a HBT does not require a negative bias voltage, enabling a single power supply operation, and can perform on/off operations similarly to a Si-MOSFET without requiring an analog switch on the drain side. HBT amplifiers also have a high output power density, and can generate a specified rated output power from a device smaller than a conventional FET power amplifier generating the same output power. HBT devices are therefore considered promising as power elements in future mobile communication devices.
Unlike FET, HBTs are current driven devices, and require a base current supply of several 10 to 100 mA to generate the high 2W to 4W output needed for use in mobile telephones conforming to Europe's GSM (Global System for Mobile Communications) standard. However, because the output current assuring a specific output voltage in a standard CMOS device is less than several mA, it is difficult to obtain such a high base current directly from a standard Si CMOS chip. A bias circuit is therefore needed to supply base current to HBT in a HBT power amplifier. Output power control is also required in mobile telephones for the GSM as well as many other communications systems, and the bias circuit must therefore also be able to adjust output power.
FIG. 8 is a circuit diagram of a conventional bias circuit in a three-stage HBT power amplifier. Referring to FIG. 8, transistors TrA1 to TrA6 are heterojunction bipolar transistors for supplying bias to a power amplifying HBT based on an input signal Vapc from a Si CMOS logic circuit such as a DA converter (not shown in the figure). Transistors TrA2, TrA4, and TrA6 can also compensate for changes in base-emitter voltage, Vbe, due to the temperature coefficient of the corresponding power amplifying HBT. Using HBT processing it is possible to form the bias circuit shown in FIG. 8 at the same time as the high frequency power amplifier, that is, the bias circuit can be integrated with the high frequency power amplifier, enabling output to be controlled according to the output voltage of the Si CMOS logic circuit.
Many current portable telephony systems also use different frequency bands for communications between a base station and the user terminal (cell phone). For example, the 900 MHz Personal Digital Cellular (PDC) cellular telephony system used in Japan uses the 940 MHz to 956 MHz band for terminal-to-base transmissions, and uses the 810 MHz to 826 MHz band for base-to-terminal transmissions. Europe's GSM 900 system, a 900 MHz band mobile telephone system, uses the 880 MHz to 915 MHz band for terminal-to-base transmissions, and uses the 935 MHz to 970 MHz band for base-to-terminal transmissions.
In communication systems that use different frequency bands for sending and receiving, noise produced in the receiving frequency band (referred to as Rx noise below) can adversely affect operation of the terminal's power amplifier during signal reception. Some means of reducing this Rx noise is therefore needed.
FIG. 9 is a graph of the Rx noise characteristic in a typical HBT power amplifier. As will be known from FIG. 9, Rx noise increases (at approximately Vapc=2.1 V) when the output power (Pout) of the power amplifier is reduced. It is to be noted that Rx noise is defined as the ratio between output power in the transmission frequency band and output power in the reception frequency band (935 MHz) of the power amplifier when a microwave signal in the transmission frequency band (915 MHz) only is input to the power amplifier.
This increase in Rx noise is unique to power amplifier HBTs. FIG. 10 is a graph of the change in gain in the transmission and reception frequency bands at a particular bias voltage in a single stage HBT power amplifier. It is to be noted that measuring Rx noise in a single stage power amplifier using the same method applied with a three-stage power amplifier is difficult due to problems relating to measurement precision. The data plotted in FIG. 10 was therefore obtained by simultaneously inputting microwave output from a synthesizer in the transmission band, and noise output from an NF measurement noise source in the reception band, to a single stage power amplifier, simultaneously measuring output in the transmission band and noise in the reception band, and expressing the results as gain. Bias was also supplied from an external rated source rather than using a bias circuit.
Referring to FIG. 10, transmission band gain Gain(Tx) drops simply in conjunction with a drop in the base voltage when power amplifier output is below the base voltage Vb (Vb&lt;1.35 V). Note that 1.35 V is the base voltage Vb at which power amplifying HBT becomes active. Gain(Rx) in the reception band, however, declines and then rises to a characteristic peak in conjunction with a drop in the base voltage. Note that because Rx noise is defined as the ratio between output power in the transmission band and output power in the reception band, this gain characteristic is thought to be attributable to an increase in Rx noise.
A problem, therefore, is that when a constant bias is supplied to the power amplifying HBT in each stage of a multistage power amplifier, the increase in noise at each stage becomes amplified and then output.